Umami

Definition: Umami

Umami
Discovered 100 years ago by a Japanese scientist, umami is believed to be a fifth taste sensation. The four basic tastes are sweet, sour, bitter, and salty. Umami is considered to be savory. This taste is found in such foods as oysters, tomatoes, and mushrooms. Difficult to recognize, umami is a subtle taste that blends well with other foods to enhance flavor.

I think I know the sensation ... tomatoes, mushrooms, and oysters all kind of make me want to gag.
From wikipedia:
http://en.wikipedia.org/wiki/Umami
A whole site about Umami:
http://www.umamiinfo.com/what_exactly_is_umami?/